1. Field of the Invention
This invention relates to a separating membrane formed of silk fibroin, a natural polypeptide, which can be advantageously used for preferential separation of water from a water-alcohol mixture so as to increase the alcohol concentration of the mixture and to a method for increasing alcohol concentration by the use of said membrane.
2. Prior Art Statement
Separation and purification of substances are indispensable operations in numerous fields of the chemical industry. The use of membranes for carrying out these operations is gaining acceptance. Separation processes using dialysis membranes, reverse osmosis membranes, and ultrafiltration membranes have been applied for example, in medical treatment, the desalination of salt water, and the production of extremely pure water. Further, oxygen-permeable membranes and other gas separation membranes are being used in some chemical processes.
While there is known a method for separating an organic substance from an aqueous solution of the organic substance by the selective permeation of the substance through a membrane, this method has not been commercially applied because it is difficult to develop a membrane capable of high-efficiency, high-speed separation. When a membrane capable of efficiently and economically effecting this separation is developed, the aforesaid method will replace such currently used separation processes such as distillation. This will result in a great energy saving and also enable simplification of production.
The separation of substances by a membrane depends on a difference in the ability of the substances to permeate through and diffuse in the membrane. More specifically, the ease with which a substance permeates a membrane increases with increasing ability of the substance for permeation and diffusion in the membrane. Thus, commercial application of the membrane for separation becomes possible when this ability becomes sufficiently high. For the purpose of selective permeation of water, for example, it suffices to form the membrane in a hydrophilic structure.
Various membranes for separating water from a water-alcohol mixture by selective permeation of water in line with the aforesaid principle have been reported. Cellulose acetate membrane [Journal of Membrane Science, Vol. 23, page 41 (1985)], bisphenol-copolymer [Polymer Journal, Vol. 17, page 499 (1985)], acrylic acid-acrylonitrile copolymer membrane [Journal of Polymer Science; Polymer Letter, Vol. 22, page 473 (1983)], N-substituted polyimide-containing copolymer membrane [Polymer Journal, Vol. 17, page 363 (1985) and ibid., Vol. 16, page 653 (1984)], chitosan membrane [Kobunshi Ronbunshu, Vol. 42, page 139 (1985)], and Nafion membrane [Journal of Membrane Science, Vol. 24, page 101 (1985)]. However, none of these membranes are adequate for practical application.